The present invention relates to a screwing apparatus for screwing parts to each other by means of screws in an assembling process.
There are demands for the development of a screwing apparatus, installed on robots or provided in assembling processes, which can be automatically operated by a rigid quality control procedure. In particular, in torque control in screwing operation, standard fastening torque is provided by the industrial standard, depending on the diameter of a screw and the material of parts to be fastened. In assembling process, it is necessary to examine whether or not parts have been screw-fastened at appropriate fastening torque. But the number of portions to be screw-fastened is so great that it is difficult to inspect all of them. Therefore, sampling inspections are performed to check whether it is possible to tighten screws by means of a torque wrench or a torque driver. In fastening parts with screws at appropriate torque, voltage-driving type drivers or current-driving type drivers are utilized to control the rotational force of a bit installed on a motor so as to fasten parts with screws at appropriate torque.
Referring to FIG. 5, the construction of a conventional screwing apparatus is described below. The screwing apparatus incorporating a brushless motor is provided with a speed reducer 2 at the bottom thereof. The rotational speed of the brushless motor is reduced by the reducer 2 and the reduced speed thereof is transmitted to a bit 3 so as to rotate a screw 10 installed at the lower end of the bit 3. The brushless motor is connected with a current-driving type driver 6 connected with a central processing unit (CPU) 4 via a D/A converter 5. Based on torque set by the CPU 4, the current-driving type driver 6 drives the brushless motor by controlling the intensity of electric current.
The operation of the screwing apparatus having the above-described construction is described below with reference to FIG. 6. The screwing process comprises a screwing region (a) between the start in screwing operation and a seating state (A) in which a seating surface 7a of a screw head 7 is brought into contact with a workpiece 8; and a fastening region (b) between the seating state (A) and the state in which the screw 10 is fastened at target fastening torque (B). In screw-sucking operation to be performed in the screwing region (a), the CPU 4 issues an instruction of a screw-sucking torque value PW1 which means a torque limit set in a low-speed rotation of the brushless motor between the state in which the screw 10 is set on a catcher 9 at a predetermined position thereof and the state in which the screw 10 is sucked by a sucking pipe 11 moved downward to cover the screw head 7 set on the catcher 9. Normally, the screw-sucking torque value PW1 is set to about 2 Kgfcm. A screwing torque value PW2 means a torque limit set in a high-speed rotation of the brushless motor in screwing operation. The screwing torque value PW2 is set to be greater than the target fastening torque (B) in view of the kind of the screw 10 and the workpiece 8 and screwing state so as to prevent the generation of a motor locking error in the screwing operation. A seating torque value PW3 is used between the state in which the screw 10 is positioned at a predetermined height 1 mm above the surface of the workpiece 8 and the state in which the screw 10 is seated on the workpiece 8. That is, the seating torque value PW3 means a torque limit at the time when the seating of the screw 34 on the workpiece 32 is detected. Normally, the seating torque value PW3 is set to be equal to torque, predetermined in the CPU 4, for deciding whether or not the screw 10 has been seated on the workpiece 8. The seating state of the screw 10 is considered in setting the seating torque value PW3. Finally, at a reversing torque value PW4, a reversing brake is operated to seat the screw 10 on the workpiece 8.
At the start in fastening the screw 10 in the fastening region (b), a fastening-start torque value PW5 is used as torque to smoothly fasten the screw 10 seated on the workpiece 8. Normally, the fastening-start torque value PW5 is set to about half as small as the seating torque value PW3. The rate (Kgfcm) of increase in torque from the fastening-start torque value PW5 until the target fastening torque (B) is set per second. For example, supposing that the increase rate of torque is set to 10, torque increases 10 Kgfcm per second. The greater the increase rate of torque is, the shorter fastening period of time is. If the increase rate of torque is too great, fastening torque overshoots, thus exceeding the target fastening torque (B).
In the above construction, during the increase in torque from the screw-sucking torque toward the target fastening torque (B), actual torque does not rise smoothly but rise rapidly due to a change in frictional force (change in dynamic frictional force, static frictional force, and slide) between the screw 10 and the workpiece 8 or a lost motion of a mechanical transmission system comprising the shaft of the motor and the speed reducer 2, even though the CPU 4 issues an instruction so that the torque of the bit 3 increases smoothly. Thus, it is difficult to fasten the screw at the target fastening torque (B). If the torque rises steeply in the vicinity of the target fastening torque (B), the fastening torque may exceed the target fastening torque (B).